1. Field of the Invention
The present invention relates generally to optical coupling of a high brightness light source to a plurality of optical light conductors and, more particularly, to a polygonal-shaped optical coupling member providing improved light mixing and thereby uniformity of light intensity and color.
2. Description of the Prior Art
Central lighting systems wherein a single light source is used to supply light output to a number of spaced apart locations using a plurality of optical light conductors, such as optical tubes, rods or fibers, have been proposed for a number of different applications including automotive, display and home lighting. For instance, U.S. Pat. No. 4,958,263 issued to Davenport et al and assigned to the same assignee as the present invention discloses the use of a central lighting system for an automotive application.
The light source for such a central lighting system must be capable of providing a light output having high brightness characteristics so as to ensure sufficient light output at the spaced apart locations. One suitable light source having a high brightness characteristic measuring in excess of 50,000 lumens per square centimeter is disclosed in the second patent application cross-referenced above. Such a light source when used in conjunction with a suitable reflector, such as one having an ellipsoidal shape, and a mirror element to focus the light output on the ends of optical fibers disposed at an optical focal plane of the reflector can produce a non-coherent high brightness focussed light output capable of causing physical damage to the ends of the optical fibers. To avoid the possible damage to the ends of the optical fibers, a light coupling member in the form of an one-piece optical cylindrical coupler rod relatively short in length is interposed between the optical focal point and the ends of the optical fibers. The light output travels through multiple internal reflections through the light coupling member from the optical focal point to the optical fibers.
Also, the non-coherent high brightness light output image produced by the light source and focussed by the ellipsoidal reflector tends to be brightest and more blue in color at the center of the light output and dimmer and more red in color around the outer edge of the light output. When the light output enters the cylindrical optical coupling member, the multiple internal reflections tend to achieve some mixing of the light and thereby reducing the differences in intensity and color from the outer edge to the center of the light output entering the optical fibers.
Although some mixing of the intensity and color of the light occurs as the light travels through the cylindrical coupling member, there is still a tendency for a bright spot to occur along the longitudinal axis of the cylindrical coupling member. The red light originating in the discharge plume of the lamp and tending to occur at the outer edge of the image at the focus of the ellipsoidal reflector tends to remain near the outer surface about the circumference of the cylindrical coupling member. Therefore, a need remains for improvement of the design of optical coupling member so as to reduce the tendency for the bright spot to occur along the longitudinal axis of the coupling member by increasing light intensity and color mixing.